This invention relates to a method and apparatus for removing an image from a window of a display and displaying only the background color. In particular, it pertains to the removing of an image from within a window of a CRT raster-scanned display device and especially to such a display when used in a computer graphics system.
The typical display device for a computer system, the CRT, is a raster-scanned device which has no inherent memory. The image that is to appear each time the raster is scanned must be generated by an outside source. Where this image is to be maintained on the screen, such as in a computer graphics system, the information is typically stored in a digital memory. Semiconductor memories are now low enough in cost that raster-scanned frame-buffer memories are commonly used. In a frame-buffer memory, each location in the memory corresponds to one picture element on the screen, hereinafter referred to by its common technical term "pixel". This type of memory allows the display hardware to be insentitive to the image content, an arbitrary image can be displayed by properly recording the data at each location in the frame-buffer memory. If it is only necessary to store the presence or absence of illumination of that particular pixel, a two-dimensional memory array is sufficient. If, however, it is desired to control the intensity of the illumination and/or the color of that particular pixel, then a three-dimensional memory is required. In the three dimensional memory, each location stores a word which is used to store the information to control the intensity and/or color of the pixel. Each time the raster on the CRT is scanned, the video signal to refresh the display is generated from the memory.
When it is desired to remove an image, which may include text and graphics from the screen, the content of the screen frame-buffer memory must be changed. In the prior art, it is possible to apply a blanking signal to the CRT to drive the entire display to black. The blanking signal is the signal that is normally applied to the CRT during the horizontal and vertical retrace periods so that the retrace does not appear on the screen. The contents of the frame-buffer memory can now be changed in the background without the user's knowledge.
Recently the use of windows has become common in order to display the results of several tasks on the screen at any one given time. A window may be defined as an area on the screen of predetermined shape into which data is written and which operates independent of the other areas on the display. In its most common form, windows are rectangular shapes which may be overlayed one on top of the other so that portions of one window may be covered by another. This provides a very cost effective technique for displaying and controlling a multiple number of tasks on a single screen.
When it is time to remove the image from one particular window, the blanking technique described above cannot be utilized because it blanks the entire screen. In addition, color monitors have now become popular. It may therefore be desirable that the window display a chosen background color rather than going to black. Presently, removing the image from a window is accomplished by erasing the data stored in the frame-buffer memory while the. frame-buffer memory is being utilized to refresh the display. When compared with the normal operating times of computer functions, this is a relatively slow operation because it commonly requires several frames to complete. Therefore, the user observes the slow erasure of the image from the screen. In addition, in some systems in which a color display is utilized, the image is not erased at a constant speed because erasing some colors requires changing more memory locations than erasing others. This operation can be annoying to the skilled user. More importantly, the inventor has discovered that it gives the user the impression that the system is unresponsive even though that it is not the case.